Presenter: Oscar Sosa (University of Puget Sound)

Description:
Homarine is one of the most abundant nitrogenous metabolites measured in the ocean. Several abundant groups of marine phytoplankton (Synechococcus and some diatoms) produce homarine as an organic osmolyte. Homarine is also widely known for its bioactive properties that affect growth and development of marine organisms. Homarine has a high potential to be a significant carbon and nitrogen currency in the microbial loop. Yet, its biochemical fate remains uncharacterized. Using seawater enrichment cultures from Puget Sound (Washington, USA) we isolated a gammaproteobacterium of the genus Cobetia, strain OBi1, which could use homarine as sole source of carbon and nitrogen. Using transcriptomics, we identified three gene clusters upregulated in cultures grown with homarine relative to cultures provided with glucose and ammonium as primary sources of carbon and nitrogen, respectively. The clusters included putative functions related to the reduction and degradation of the uracil ring, a pathway for sarcosine (N-methylglycine) oxidation, and flagellar biosynthesis. These results suggest that homarine catabolism proceeds with the degradation of its pyridine ring catalyzed by enzymes related to the uracil degradation pathway, followed by formation of sarcosine and an uncharacterized hydrocarbon. Sarcosine oxidation transfers the methyl group to the tetrahydrofolate pool and produces glycine. OBi1 presumably used glycine as its primary nitrogen source. We also detected upregulation of a putative glycine betaine transporter, likely responsible for homarine uptake. These results demonstrate that homarine can provide carbon, energy, and nitrogen to marine bacteria. Homarine seems to induce genes for its catabolism and for motility, consistent with the need of bacteria to find hot spots of organic nutrients derived from phytoplankton. The genes identified in OBi1 will allow us to identify additional microorganisms with the potential to catabolize homarine.